Translational Brownian motion in a fluid with internal degrees of freedom

The friction coefficient for a smooth hard sphere (Brownian particle) moving through a fluid composed of finite-sized particles with internal rotational degrees of freedom is computed. Inertial effects are included. The long-time limit of the velocity-autocorrelation function is obtained. We find that the velocity-autocorrelation function decays as $t^{-\frac{3}{2}}$ but with a coefficient that is altered from the usual Navier-Stokes results to include contributions from transport processes involving internal rotational degrees of freedom of the fluid.